Much of the world's production of soda ash is produced from natural trona deposits. Natural trona ore is a hydrated mixture of sodium carbonate and sodium bicarbonate along with various organic and inorganic impurities. Currently, soda ash is produced from trona by one of two processes (1) the monohydrate process or (2) the sesquicarbonate process.
In the monohydrate process, trona ore is first calcined in a rotary kiln at temperatures of 175 to 200.degree. C./347 to 397.degree. F. This serves to convert bicarbonate to carbonate. Calcining operations at temperatures between 350 to 400.degree. C. also destroy organic impurities present in the ore. Inorganic contaminants are removed from the calcined trona by dissolving the material in water and recrystallizing sodium carbonate from the filtered solution through the use of heat applied to the water. Soluble inorganic impurities, such as sodium carbonate and sodium sulfate, remain in the mother liquor. Insoluble impurities, such as shale and calcium carbonate, are removed by filtration prior to crystallization. The resulting sodium carbonate crystals, in the monohydrate form, are separated by filtration or centrifugation. The monohydrate crystals are then dried and calcined to anhydrous sodium carbonate.
The sesquicarbonate process utilizes basically the same unit operations as the monohydrate process. However, the arrangement of these unit operations differs. In the sesquicarbonate process, trona ore is first dissolved in hot water and the resulting solution filtered to remove insoluble impurities. Organic impurities are then removed by adsorption of the organics on activated carbon. Pure trona (or sesquicarbonate) is then recrystallized from the purified solution by using triple-effect evaporators. A solution of sodium carbonate (to maintain in excess of 10 to 25% excess carbonate) is recycled in the evaporators so as to obtain the sesquicarbonate. Since trona is an incongruently dissolving double salt, sesquicarbonate cannot be formed by cooling. This, once again, leaves soluble inorganic impurities in the mother liquor. The sesquicarbonate crystals are then calcined to produce sodium carbonate.
These processes are described in detail in various U.S. patents. For example, U.S. Pat. No. 3,479,133, issued on Nov. 18, 1969, to F. M. Warzel describes the monohydrate process. U.S. Pat. No. 3,119,655, issued in January of 1964, to Frint et al. describes the sesquicarbonate process. Similarly, U.S. Pat. No. 3,260,567, issued on July of 1966, to Hellmers et al. and U.S. Pat. No. 3,361,540, issued on Jan. 2, 1968, to Peverly et al. teach these sesquicarbonate processes.
Both the monohydrate and sesquicarbonate processes produce sodium carbonate crystals having a density range of 0.95 to 1.25 g/cc. Some applications (those in which the sodium carbonate is to be used in solution form) prefer the use of lower density crystals or higher surface area crystals. U.S. Pat. No. 5,043,149, issued on Aug. 27, 1991, to Frint et al., and assigned to the FMC Corporation, describes a process for the manufacture of such low density soda ash crystals. Sodium carbonate crystals obtained from all of the above process will vary greatly in size distribution. A large variety of commercial products are produced by the above-described processes. Each of the sodium carbonate crystals formed by these various processes were analyzed for the purpose of showing the size distribution of the crystals. The attached Table I shows the size distribution and shape of the various commercial products:
TABLE I __________________________________________________________________________ Commercial Products General Chemical ITOCHU Chem Solution ID FMC 100 FMC 160 FMC 260 RP Lite RP Dense Synthetic Fine Synthetic __________________________________________________________________________ MeOH Feed rate N/A N/A N/A N/A N/A N/A N/A RP feed rate N/A N/A N/A N/A N/A N/A N/A MeOH feed rate N/A N/A N/A N/A N/A N/A N/A Initial soln. volume RPM N/A N/A N/A N/A N/A N/A N/A Location of addition N/A N/A N/A N/A N/A N/A N/A Crystal Density (lb/ft3) 49.4 61.9 65.31 52 61.4 35 60.2 Size Distribution (%) 1000 u 0.2 850 u 0 0 0 2.8 0.3 0 3.6 600 u 0.03 8.5 425 u 0.98 21.1 355 u 15.5 10.1 11.38 28.5 54.7 4.1 300 u 23.3 250 u 30.4 35.8 29.39 31.7 33.2 4.3 212 u 24.5 150 u 29.48 13.5 106 u 46.4 49.2 17.21 32.8 11 35.2 3.1 75 u 7.55 0.9 63 u 7.6 4.6 2.71 4.1 0.8 33.1 45 u 1 &lt;45 u 0.3 38 u 0.1 0.2 0.1 0.1 14.5 &lt;38 u 0 0 0 0 8.5 Screened x x x x x x x Not Screened x Detergency (%) Absorptivity (%) 13.9 12.5 25 17.6 Sulfate (ppm) 300 400 700 1000 1000 200 600 TOC (ppm) 56 2 8 Crystal Morphology Large rods Small rods Blocky Mixed balls Mixed balls Small snowflakes Small Balls Date 6/26/95 6/28/95 9/1/95 6/28/95 6/28/95 6/28/95 Nov. 95 __________________________________________________________________________
The various sizes, shapes and distributions of crystals are applicable in various processes. For example, a large size distribution can adversely affect the dissolving rates of the sodium carbonate and also can produce undesirable dust (at less than about 60 microns). This poses a problem if the material is to be used in dry processes, such as glass manufacturing. In addition, a wide particle size distribution can cause serious problems in the filtration or centrifugation processes which are used to separate the crystals from the mother liquor. As such, it is desirable to form sodium carbonate crystals which have a size distribution, shape and density which mirrors that of commercial products while producing such products at a relatively low cost.
U.S. Pat. No. 4,584,077, issued on Apr. 22, 1986, to Chlanda et al. describes a process for recovering sodium carbonate from trona and other mixtures of sodium carbonate and sodium bicarbonate. This process includes the steps of: (1) forming an aqueous solution comprising sodium carbonate and sodium bicarbonate; (2) removing a portion of the sodium bicarbonate from the solution so as to form a mother liquor comprising sodium carbonate and a reduced amount of sodium bicarbonate; (3) subjecting the mother liquor to an electrodialytic water splitting by circulating the water liquor through an electrodialytic water splitter to produce a liquid reaction product comprising sodium carbonate substantially free of sodium bicarbonate; and (4) withdrawing the liquid reaction product comprising sodium carbonate substantially free of sodium bicarbonate from the electrodialytic water splitter. In this patent, it was described that the sodium carbonate solution product from the base compartment is fed to a primary absorber wherein a liquid loading substance is absorbed into the sodium carbonate solution. The "liquid loading substance" includes liquids such as ammonia, methanol, ethanol and the like. This is added to the sodium carbonate solution to cause the sodium carbonate to crystallize out as the decahydrate, monohydrate or mixtures thereof. As a reaction product, these can be readily separated from the crystallized sodium carbonate-containing material.
This Chlanda process is an extremely energy inefficient process for producing sodium carbonate from trona. A sodium carbonate solution is produced from an electrodialytic water splitter. Sodium bicarbonate is converted to sodium carbonate prior to reacting with the "liquid loading substance".
It is an object of the present invention to provide a method for the manufacture of sodium carbonate or bicarbonate crystals that is cost effective.
It is another object of the present invention to provide a process for the manufacture of sodium carbonate or bicarbonate from trona or other bicarbonate and/or carbonate minerals or solutions that allows for an improved control of crystal size, shape, and density without the use of organic or inorganic additive agents.
It is another object of the present invention to provide a process that can be used on any bicarbonate, carbonate or bicarbonate-carbonate mixture.
It is still a further object of the present invention to provide a process that allows for the controlled production of bicarbonates and carbonates.
It is another and further object of the present invention to provide a process that controls crystal size, shape and density for carbonate precipitated from carbonate solution and for bicarbonate/carbonate mixtures.
It is still a further object of the present invention to provide a precipitation step that allows for the conversion of bicarbonate to carbonate or carbonate to bicarbonate for certain ratios of carbonate to bicarbonate.
It is still another object of the present invention to provide for wet calcining of trona or other bicarbonate/carbonate mixtures to carbonate through the use of steam stripping.
It is still a further object of the present invention to provide a process whereby the aqueous solution is added to the methanol.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.